The invention set forth in this specification pertains to new and improved infrared thermometers which are primarily intended for use in making stress-degree measurements for irrigation purposes.
So-called infrared thermometers are in effect temperature measuring apparatuses or devices for use in measuring the temperature of an object or a series of closely adjacent objects through the measuring of relative infrared radiation levels by the use of an infrared detector--frequently a thermopile. In these devices a circuit is used to convert the signal produced by the infrared detector and associated circuitry such as, for example, an amplifier and a linearization circuit, into an output signal capable of indicating a numerical value. This output signal is then utilized in a meter type structure such as a digital display or a known meter movement in order to indicate a numerical temperature value.
The temperature indicated corresponds to the temperature of an object or a series of objects as indicated by the relative levels of radiation of such an object or series of objects and of the infrared detector itself. Such infrared thermometers are considered to be very desirable for many purposes. They normally are constructed as hand-held, gun-like instruments which can be easily and conveniently used.
Unfortunately such infrared thermometers do not fulfill a current need for devices which can simply and easily make stress-degree measurements as are required or desirable for a number of agricultural applications. These measurements require a comparison of the temperature of a plant or of a crop canopy with the ambient temperature adjacent to the plant or crop canopy. Such a comparison is desirable in determining whether or not a plant or crop is "stressed" in the sense that it is not transpiring a desired amount of water vapor.
Normally the transpiration of water vapor by a plant lowers the temperature of the plant with respect to the surrounding air temperature. Normally the amount of water that a plant transpires is related to the amount of moisture available to the root system of the plant. Hence, the need for watering or irrigating a plant or a crop can be determined by the differential between the temperature of the plant or crop and the temperature of the ambient air. This differential may be expressed in temperature--degrees or stress-degrees.
Such stress-degrees are normally considered with reference to the number of days when a plant is experiencing moisture stress as the result of inadequate moisture being available to the roots of a plant so as to obtain a composite figure indicating when a plant or crop should be watered or irrigated. The resultant stress-degree-day figure as obtained with reference to the temperature differential between the plant or crop and the ambient air on a number of successive days is important for irrigation and similar watering purposes. Although plants can withstand comparatively restricted periods of a comparatively large amount of moisture stress, the total amount of moisture stress experienced by a plant or crop over a period can have significant effects upon plant growth or yield.
The need for new and improved infrared thermometers in accordance with the invention for making stress-degree measurements is related to the fact that such measurements should be capable of being easily and conveniently made. In the past such stress-degree measurements have been made utilizing both known infrared thermometers and conventional ambient air measuring devices such as conventional mercury thermometers and the like. Such use of two different items of measuring equipment to obtain stress-degree measurements is disadvantageous. With such use of two different items of measuring equipment a limited amount of arithmetic is required. Such arithmatic computations as required while not significantly difficult for many individuals, have proved undesirable because of the frequency of errors. Frequently those employed to make stress-degree measurements can be best described as not being overly literate.